Modified preamble for programmable transmitter

ABSTRACT

A programmable transmitter generates a frame preamble to train a receiver with respect to a communication link format that corresponds to a transmission mode wherein the transmission mode may comprise transmitting the communication link over one or more antennas. Generally, the invention includes generating a preamble with an arrangement that depends upon whether a Greenfield (high data rate) or mixed mode transmission is to occur and that depends upon a number of spatial streams that are to be generated. One format for high data rate transmission includes a short training sequence, a long training sequence and a signal field. The mixed mode transmission further includes a legacy prefix.

CROSS REFERENCE TO RELATED PATENTS

This invention is claiming priority under 35 USC § 119(e) to aprovisionally filed patent application having the same title as thepresent patent application, a filing date of Jun. 7, 2005, and anapplication number of 60/689,932.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates generally to wireless communication systems andin particular to a transmitter operating at high data rates within suchwireless communication systems.

2. Description of Related Art

Communication systems are known to support wireless and wire linedcommunications between wireless and/or wire lined communication devices.Such communication systems range from national and/or internationalcellular telephone systems to the Internet to point-to-point in-homewireless networks. Each type of communication system is constructed, andhence operates, in accordance with one or more communication standards.For instance, wireless communication systems may operate in accordancewith one or more standards including, but not limited to, IEEE 802.11,Bluetooth, advanced mobile phone services (AMPS), digital AMPS, globalsystem for mobile communications (GSM), code division multiple access(CDMA), local multi-point distribution systems (LMDS),multi-channel-multi-point distribution systems (MMDS), and/or variationsthereof.

Depending on the type of wireless communication system, a wirelesscommunication device, such as a cellular telephone, two-way radio,personal digital assistant (PDA), personal computer (PC), laptopcomputer, home entertainment equipment, et cetera communicates directlyor indirectly with other wireless communication devices. For directcommunications (also known as point-to-point communications), theparticipating wireless communication devices tune their receivers andtransmitters to the same channel or channels (e.g., one of the pluralityof radio frequency (RF) carriers of the wireless communication system)and communicate over that channel(s). For indirect wirelesscommunications, each wireless communication device communicates directlywith an associated base station (e.g., for cellular services) and/or anassociated access point (e.g., for an in-home or in-building wirelessnetwork) via an assigned channel. To complete a communication connectionbetween the wireless communication devices, the associated base stationsand/or associated access points communicate with each other directly,via a system controller, via the public switch telephone network, viathe Internet, and/or via some other wide area network.

For each wireless communication device to participate in wirelesscommunications, it includes a built-in radio transceiver (i.e., receiverand transmitter) or is coupled to an associated radio transceiver (e.g.,a station for in-home and/or in-building wireless communicationnetworks, RF modem, etc.). As is known, the receiver is coupled to theantenna and includes a low noise amplifier, one or more intermediatefrequency stages, a filtering stage, and a data recovery stage. The lownoise amplifier receives inbound RF signals via the antenna andamplifies then. The one or more intermediate frequency stages mix theamplified RF signals with one or more local oscillations to convert theamplified RF signal into baseband signals or intermediate frequency (IF)signals. The filtering stage filters the baseband signals or the IFsignals to attenuate unwanted out of band signals to produce filteredsignals. The data recovery stage recovers raw data from the filteredsignals in accordance with the particular wireless communicationstandard.

As is also known, the transmitter includes a data modulation stage, oneor more intermediate frequency stages, and a power amplifier. The datamodulation stage converts raw data into baseband signals in accordancewith a particular wireless communication standard. The one or moreintermediate frequency stages mix the baseband signals with one or morelocal oscillations to produce RF signals. The power amplifier amplifiesthe RF signals prior to transmission via an antenna.

Typically, the transmitter will include one antenna for transmitting theRF signals, which are received by a single antenna, or multipleantennas, of a receiver. When the receiver includes two or moreantennas, the receiver will select one of them to receive the incomingRF signals. In this instance, the wireless communication between thetransmitter and receiver is essentially a single-input-single-output(SISO) communication, even if the receiver includes multiple antennasthat are used as diversity antennas (i.e., selecting one of them toreceive the incoming RF signals). For SISO wireless communications, atransceiver includes one transmitter and one receiver. Currently, mostwireless local area networks (WLAN) that are IEEE 802.11, 802.11a,802,11b, or 802.11g employ SISO wireless communications.

Other types of wireless communications includesingle-input-multiple-output (SIMO), multiple-input-single-output(MISO), and multiple-input-multiple-output (MIMO). In a SIMO wirelesscommunication, a single transmitter processes data into radio frequencysignals that are transmitted to a receiver. The receiver includes two ormore antennas and two or more receiver paths. Each of the antennasreceives the RF signals and provides them to a corresponding receiverpath (e.g., LNA, down conversion module, filters, and ADCs). Each of thereceiver paths processes the received RF signals to produce digitalsignals, which are combined and then processed to recapture thetransmitted data.

For a multiple-input-single-output (MISO) wireless communication, thetransmitter includes two or more transmission paths (e.g., digital toanalog converter, filters, up-conversion module, and a power amplifier)that each converts a portion of baseband signals into RF signals, whichare transmitted via corresponding antennas to a receiver. The receiverincludes a single receiver path that receives the multiple RF signalsfrom the transmitter.

For a multiple-input-multiple-output (MIMO) wireless communication, thetransmitter and receiver each include multiple paths. In such acommunication, the transmitter parallel processes data using a spatialand time encoding function to produce two or more streams of data. Thetransmitter includes multiple transmission paths to convert each streamof data into multiple RF signals. The receiver receives the multiple RFsignals via multiple receiver paths that recapture the streams of datautilizing a spatial and time decoding function. The recaptured streamsof data are combined and subsequently processed to recover the originaldata.

With the various types of wireless communications (e.g., SISO, MISO,SIMO, and MIMO), it would be desirable to use one or more types ofwireless communications to enhance data throughput within acommunication system. For example, high data rates can be achieved withMIMO communications in comparison to SISO communications. However, mostcommunication systems include legacy wireless communication devices(i.e., devices that are compliant with an older version of a wirelesscommunication standard). As such, a transmitter capable of MIMO wirelesscommunications should also be backward compatible with legacy devices tofunction in a majority of existing communication systems.

In addition to the different type of wireless communications (e.g.,SISO, SIMO, MISO, and MIMO), the channel bandwidth varies from standardto standard. For example, IEEE 802.11 (j) prescribes a 10 MHz channelbandwidth, IEEE 802.11(a) and (g) prescribe a 20 MHz channel, and IEEE802.11(n) is contemplating a channel bandwidth of 40 MHz. Accordingly,for a radio to be compliant with one or more of these standards, theradio transmitter must be adjustable to accommodate the differentchannel bandwidths and transmission modes.

Therefore, a need exists for a programmable transmitter that is capableof high data throughput, backward compatible with legacy devices andadjustable to different channel bandwidths. A need further exists for amodified pre-amble that supports high data rate and mixed-modetransmissions that are detectable by legacy devices.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to apparatus and methods of operationthat are further described in the following Brief Description of theDrawings, the Detailed Description of the Invention, and the claims.Other features and advantages of the present invention will becomeapparent from the following detailed description of the invention madewith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a wireless communication systemin accordance with the present invention;

FIG. 2 is a schematic block diagram of a wireless communication devicein accordance with the present invention;

FIGS. 3A and 3B are a schematic block diagram of a radio transmitter inaccordance with the present invention;

FIG. 4 is a diagram illustrating the generation of time domain trainingsymbols from frequency domain training symbols using the programmabletransmitter of FIG. 3 in accordance with embodiments of the presentinvention;

FIG. 5 is a diagram illustrating the generation of training symbols inthe time domain using the programmable transmitter of FIG. 5 inaccordance with the present invention;

FIGS. 6 and 7 are illustrations of exemplary embodiments of theinvention of high data rate and mixed mode preamble signal formats; and

FIG. 8 is a flow chart that illustrates an embodiment of the inventionfor a method for generating a preamble either for high data rate ormixed mode transmissions over one or more spatial streams.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram illustrating a communication system10 that includes a plurality of base stations and/or access points12-16, a plurality of wireless communication devices 18-32 and a networkhardware component 34. The wireless communication devices 18-32 may belaptop host computers 18 and 26, personal digital assistant hosts 20 and30, personal computer hosts 24 and 32 and/or cellular telephone hosts 22and 28. The details of the wireless communication devices will bedescribed in greater detail with reference to FIG. 2.

The base stations or access points 12-16 are operably coupled to thenetwork hardware 34 via local area network connections 36, 38 and 40.The network hardware 34, which may be a router, switch, bridge, modem,system controller, et cetera, provides a wide area network connection 42for the communication system 10. Each of the base stations or accesspoints 12-16 has an associated antenna or antenna array to communicatewith the wireless communication devices in its area. Typically, thewireless communication devices register with a particular base stationor access point 12-14 to receive services from the communication system10. For direct connections (i.e., point-to-point communications),wireless communication devices communicate directly via an allocatedchannel.

Typically, base stations are used for cellular telephone systems andlike-type systems, while access points are used for in-home orin-building wireless networks. Regardless of the particular type ofcommunication system, each wireless communication device includes abuilt-in radio and/or is coupled to a radio.

FIG. 2 is a schematic block diagram illustrating a wirelesscommunication device that includes the host device 18-32 and anassociated radio 60. For cellular telephone hosts, the radio 60 is abuilt-in component. For personal digital assistants hosts, laptop hosts,and/or personal computer hosts, the radio 60 may be built-in or anexternally coupled component.

As illustrated, the host device 18-32 includes a processing module 50,memory 52, radio interface 54, input interface 58 and output interface56. The processing module 50 and memory 52 execute the correspondinginstructions that are typically done by the host device. For example,for a cellular telephone host device, the processing module 50 performsthe corresponding communication functions in accordance with aparticular cellular telephone standard.

The radio interface 54 allows data to be received from and sent to theradio 60. For data received from the radio 60 (e.g., inbound data), theradio interface 54 provides the data to the processing module 50 forfurther processing and/or routing to the output interface 56. The outputinterface 56 provides connectivity to an output display device such as adisplay, monitor, speakers, et cetera such that the received data may bedisplayed. The radio interface 54 also provides data from the processingmodule 50 to the radio 60. The processing module 50 may receive theoutbound data from an input device such as a keyboard, keypad,microphone, et cetera via the input interface 58 or generate the dataitself. For data received via the input interface 58, the processingmodule 50 may perform a corresponding host function on the data and/orroute it to the radio 60 via the radio interface 54.

Radio 60 includes a host interface 62, a baseband processing module 64,memory 66, a plurality of radio frequency (RF) transmitters 68-72, atransmit/receive (T/R) module 74, a plurality of antennas 82-86, aplurality of RF receivers 76-80 and a local oscillation module 99. Thebaseband processing module 64, in combination with operationalinstructions stored in memory 66, executes digital receiver functionsand digital transmitter functions, respectively. The digital receiverfunctions include, but are not limited to, digital intermediatefrequency to baseband conversion, demodulation, constellation demapping,decoding, de-interleaving, fast Fourier transform, cyclic prefixremoval, space and time decoding, and/or descrambling. The digitaltransmitter functions include, but are not limited to, scrambling,encoding, interleaving, constellation mapping, modulation, inverse fastFourier transform, cyclic prefix addition, space and time encoding, anddigital baseband to IF conversion. The baseband processing module 64 maybe implemented using one or more processing devices. Such a processingdevice may be a microprocessor, micro-controller, digital signalprocessor, microcomputer, central processing unit, field programmablegate array, programmable logic device, state machine, logic circuitry,analog circuitry, digital circuitry, and/or any device that manipulatessignals (analog and/or digital) based on operational instructions. Thememory 66 may be a single memory device or a plurality of memorydevices. Such a memory device may be a read-only memory, random accessmemory, volatile memory, non-volatile memory, static memory, dynamicmemory, flash memory, and/or any device that stores digital information.Note that when the processing module 64 implements one or more of itsfunctions via a state machine, analog circuitry, digital circuitry,and/or logic circuitry, the memory storing the corresponding operationalinstructions is embedded with the circuitry comprising the statemachine, analog circuitry, digital circuitry, and/or logic circuitry.

In operation, the radio 60 receives outbound data 88 from the hostdevice via the host interface 62. The baseband processing module 64receives the outbound data 88 and, based on a mode selection signal 101,produces one or more outbound symbol streams 90. The mode selectionsignal 101 indicates a particular mode of operation that is compliantwith one or more specific modes of the various IEEE 802.11 standards.For example, the mode selection signal 101 may indicate a frequency bandof 2.4 GHz, a channel bandwidth of 20 or 22 MHz and a maximum bit rateof 54 megabits-per-second. In this general category, the mode selectionsignal will further indicate a particular rate ranging from 1megabit-per-second to 54 megabits-per-second. In addition, the modeselection signal 101 may indicate a particular type of modulation, whichincludes, but is not limited to, Barker Code Modulation, BPSK, QPSK,CCK, 16 QAM and/or 64 QAM. The mode select signal 102 may also include acode rate, a number of coded bits per subcarrier (NBPSC), coded bits perOrthogonal Frequency Division Multiplexing (OFDM) symbol (NCBPS), and/ordata bits per OFDM symbol (NDBPS). The mode selection signal 101 mayalso indicate a particular channelization for the corresponding modethat provides a channel number and corresponding center frequency. Themode select signal 102 may further indicate a power spectral densitymask value and a number of antennas to be initially used for a SISO,SIMO, MISO or MIMO communication, and a corresponding space-time and/orspace-frequency encoding mode.

The baseband processing module 64, based on the mode selection signal101 produces one or more outbound symbol streams 90 from the outbounddata 88. For example, if the mode selection signal 101 indicates that asingle transmit antenna is being utilized for the particular mode thathas been selected, the baseband processing module 64 will produce asingle outbound symbol stream 90. Alternatively, if the mode selectsignal 102 indicates 2, 3 or 4 antennas, the baseband processing module64 will produce 2, 3 or 4 outbound symbol streams 90 from the outbounddata 88.

Depending on the number of outbound streams 90 produced by the basebandmodule 64, a corresponding number of the RF transmitters 68-72 will beenabled to convert the outbound symbol streams 90 into outbound RFsignals 92. In general, each of the RF transmitters 68-72 includes adigital filter and upsampling module, a digital to analog conversionmodule, an analog filter module, a frequency up conversion module, apower amplifier, and a radio frequency bandpass filter. The RFtransmitters 68-72 provide the outbound RF signals 92 to thetransmit/receive module 74, which provides each outbound RF signal to acorresponding antenna 82-86.

When the radio 60 is in the receive mode, the transmit/receive module 74receives one or more inbound RF signals 94 via the antennas 82-86 andprovides them to one or more RF receivers 76-80. The RF receiver 76-80converts the inbound RF signals 94 into a corresponding number ofinbound symbol streams 96. The number of inbound symbol streams 96 willcorrespond to the particular mode in which the data was received. Thebaseband processing module 64 converts the inbound symbol streams 96into inbound data 98, which is provided to the host device 18-32 via thehost interface 62.

As one of average skill in the art will appreciate, the wirelesscommunication device of FIG. 2 may be implemented using one or moreintegrated circuits. For example, the host device may be implemented onone integrated circuit, the baseband processing module 64 and memory 66may be implemented on a second integrated circuit, and the remainingcomponents of the radio 60, less the antennas 82-86, may be implementedon a third integrated circuit. As an alternate example, the radio 60 maybe implemented on a single integrated circuit. As yet another example,the processing module 50 of the host device and the baseband processingmodule 64 may be a common processing device implemented on a singleintegrated circuit. Further, the memory 52 and memory 66 may beimplemented on a single integrated circuit and/or on the same integratedcircuit as the common processing modules of processing module 50 and thebaseband processing module 64.

FIGS. 3A and 3B illustrate a more detailed schematic block diagram of anexemplary multiple transmit path transmitter (e.g., including basebandprocessing module 64 and RF transmitters 68-72 of the radio transceiver60 of FIG. 2) using Orthogonal Frequency Division Multiplexing (OFDM) inaccordance with the present invention. FIG. 3A illustrates an exemplarybaseband processing portion 100 of a transmitter, while FIG. 3Billustrates an exemplary radio portion 130 of the transmitter. In FIG.3A, the baseband processing portion 100 is shown to include a scrambler102, channel encoder 104, a plurality of interleavers 106-108, aplurality of symbol mappers 110-112, a plurality of tone mappers113-115, plurality of inverse fast Fourier transform (IFFT)/cyclicprefix addition modules 116-120 and plurality of digitalfilter/up-sampling modules 124-128. The baseband portion of thetransmitter 100 may further include a mode manager module 105 thatreceives the mode selection signal 101 and produces a rate and transmitmode selection signal 101 for the baseband portion of the transmitter.

In operation, the scrambler 102 adds a pseudo random sequence to theoutbound data bits 88 to make the data appear random. A pseudo randomsequence may be generated from a feedback shift register with thegenerator polynomial of S(x)=x⁷+x⁴+1 to produce scrambled data. Thechannel encoder 104 receives the scrambled data and generates a newsequence of bits with redundancy. This will enable improved detection atthe receiver. The channel encoder 104 may operate in one of a pluralityof modes. For example, in IEEE 802.11(a) and IEEE 802.11(g), the channelencoder has the form of a rate 1/2 convolutional encoder with 64 statesand a generator polynomials of G₀=133₈ and G₁=101₈. The output of theconvolutional encoder may be punctured to rates of 1/2, 2/3 and 3/4according to specified rate tables. For backward compatibility with IEEE802.11(b) and the CCK modes of IEEE 802.11(g), the channel encoder hasthe form of a CCK code as defined in IEEE 802.11(b). For higher datarates, the channel encoder may use the same convolution encoding asdescribed above or it may use a more powerful code, including aconvolutional code with more states, a parallel concatenated (turbo)code and/or a low density parity check (LDPC) block code. Further, anyone of these codes may be combined with an outer Reed Solomon code.Based on a balancing of performance, backward compatibility and lowlatency, one or more of these codes may be optimal. In otherembodiments, there may be multiple channel encoders, instead of thesingle channel encoder 104 shown in FIG. 3A.

The channel encoder 104 further converts the serial encoded data streaminto M-parallel streams for transmission and provides the M-parallelstreams to interleavers 106-108. The interleavers 106-108 receive theencoded data streams and spread the encoded data streams over multiplesymbols and multiple transmit paths. This allows improved detection anderror correction capabilities at the receiver. In one embodiment, theinterleavers 106-108 follow the IEEE 802.11(a) or (g) standard in thebackward compatible modes. In another embodiment, the interleavers106-108 follow the IEEE 802.11(n) standard. In other embodiments, theremay be different configurations of the encoder/scrambler/interleaver,such as combinations of single or multiple interleavers, single ormultiple encoders, single or multiple scramblers and single or multiplespatial demultiplexers for demultiplexing the serial data stream intoM-parallel streams.

Each symbol mapper 110-112 receives a corresponding one of theM-parallel paths of data from interleavers 106-108. Each tone mapper113-115 maps bit streams to quadrature amplitude modulated QAM symbols(e.g., BPSK, QPSK, 16 QAM, 64 QAM, 256 QAM, et cetera) for each tone ofan OFDM channel according to a specific rate table. For IEEE 802.11(a)backward compatibility, double gray coding may be used. The QAM symbolsfor each tone collectively form a frequency domain OFDM symbol. Eachtone mapper 113-115 generate the tones (e.g., subcarriers of an OFDMchannel) for a particular transmit antenna, in which each tone containsa sequence of QAM frequency domain symbols. This may also include emptyguard tones or pilot tones, i.e., tones known to the receiver.

The complex QAM tone amplitudes produced by each of the tone mappers113-115 are provided to the IFFT/cyclic prefix addition modules 116-120,which perform frequency domain to time domain conversions and optionallyadd a prefix, which allows removal of inter-symbol interference at thereceiver. For example, a 64-point IFFT can be used for 20 MHz channelsand 128-point IFFT can be used for 40 MHz channels. The output of theIFFTs 116-120 are respective time domain OFDM symbols to be transmittedin a respective channel. Each time domain OFDM symbol is a superpositionof the time domain QAM symbols for each of the tones. The digitalfiltering/up-sampling modules 124-128 filter the corresponding symbolsand adjust the sampling rates to correspond with the desired samplingrates of the radio portion of the transmitter 100.

FIG. 3B illustrates the radio portion 130 of the transmitter 100 thatincludes a plurality of digital-to-analog conversion modules 140-144,analog filters 146-156, I/Q modulators 158-162, RF amplifiers 164-168,RF filters 170-174 and antennas 176-180. The M-outputs from the digitalfiltering/up-sampling modules 124-128 are received by respectivedigital-to-analog conversion modules 140-144. In operation, the numberof radio paths that are active correspond to the number of M-outputs.For example, if only one M-output path is generated, only one of theradio transmitter paths will be active. As one of average skill in theart will appreciate, the number of output paths may range from one toany desired number.

The digital-to-analog conversion modules 140-144 convert the digitalfiltered and up-sampled signals into corresponding in-phase andquadrature analog signals. The analog filters 146-156 filter thecorresponding in-phase and/or quadrature components of the analogsignals, and provide the filtered signals to the corresponding I/Qmodulators 158-162. The I/Q modulators 158-162 based on a localoscillation, which is produced by a local oscillator 99, up-converts theI/Q signals into radio frequency signals. The RF amplifiers 164-168amplify the RF signals which are then subsequently filtered via RFfilters 170-174 before being transmitted via antennas 176-180.

FIG. 4 is a diagram illustrating the generation of time domain OFDMtraining samples from frequency domain QAM training symbols given pertone in accordance with the present invention. In FIG. 4, the frequencydomain QAM training symbol 250 for each tone obtained for a trainingsequence segment. In one system used with the embodiments of theinvention, tone map tables provide the tones that are input to the IFFT254 to convert all of the frequency domain QAM training symbols 250 to atime domain OFDM training symbol 258 containing a number of samples ofsymbols 250. Using the starting sample 264 and length 268 in the framestructure table, the readout module 272 generates the complete timedomain OFDM training symbol at the indicated length 268 and beginning atthe indicated starting sample 264. For example, as shown in FIG. 4, eachfrequency domain training segment has a length of 128 symbol samples 250and the time domain OFDM training symbol 258 (which is a superpositionof the time domain QAM symbols per tone) has an initial length of 128samples. The readout module 272 begins at sample 96 and reads out thesamples in order wrapping around to the first sample until the length ofthe time domain symbol sequence equals the specified length 268. Thereadout module 272 further scales the time domain OFDM training symbol258 by the scaling factor 280 in the frame structure tables. The scalingfactor is applied to the complete time domain OFDM training symbol 258to scale the magnitude of the symbol, and the scaled time domain OFDMtraining symbol 276 is output by the readout module 272. It should benoted that in the embodiment shown in FIG. 4, the number of samples tobe generated is given by two times the number stored in the LenX2 field268 to save one bit in the bitwidth of each sub-entry of a framestructure table. However, in other embodiments, the length (total numberof samples requested for a given frame segment) could be specifieddirectly in the length field 268, i.e., without the factor 2modification.

FIG. 4 illustrates but one embodiment of performing a cyclic shift forthe purpose of avoiding signal cancellation due to the formation ofnulls that may occur without such cyclic shifts due to signal alignment.It should be understood that other variations or specific steps forperforming a cyclic shift may utilized without departing from the scopeof the teachings herein for the various embodiments of the invention.Further, such cyclic shifts may be performed on any portion of thepreamble including, as in the embodiments of the invention, on the shortand long training sequences.

FIG. 5 is a diagram illustrating the generation of a sequence of OFDMtraining symbols in the time domain in accordance with the presentinvention. In FIG. 5, the time domain OFDM training symbols 284 areobtained from the time domain time-domain sequence tables and input to atime-domain sequence engine 288. Using the starting sample 264 andlength 268 in a frame structure table, the time-domain sequence engine288 generates the complete sequence of time domain OFDM training symbolsat the indicated length 268 and beginning at the indicated startingsample 264. For example, as shown in FIG. 5, the sequence time domainOFDM training symbols 284 have an initial length of 16 samples. Thetime-domain sequence engine 288 begins at sample 8 and reads out thesamples in order wrapping around to the first sample until the length ofthe time domain OFDM symbol sequence equals the specified length 268.Again, in this example and corresponding embodiment, the true length innumber of samples to be generated by the time domain sequence engine isgiven by two times LenX2, that it, 2*80=100 overall samples. Thetime-domain sequence engine 288 further scales the time domain OFDMtraining symbols by the scaling factor 292 stored in the frame structuretables. The scaling factor is applied to the complete sequence of timedomain OFDM training symbols to scale the magnitude of the symbols, andthe scaled time domain OFDM training symbols 296 are output by thetime-domain sequence engine 288.

The above described embodiments of the invention or alternativestherefor, may also be used to provide a cyclic shift for any portion ofa high data rate or mixed mode signal field. Additionally, it isdesirable to provide a uniform approach to generating preambles fordifferent transmission modes. For example, in a high data ratetransmission mode, a so-called Greenfield mode, a basic formationincludes a 24 microsecond pre-amble for transmission of only one streamand an additional 4 microseconds for each additional stream. In a legacyor mixed mode, a pre-amble is allocated 36 microseconds for transmissionof only one stream and an additional 4 microseconds for each additionalstream. The number of streams equals the number of outputs from STBCencoder. The short training sequence (SS) is 20 MHz and 40 MHz for theGreenfield. For this embodiment, beam forming, if applied, covers theentire pre-amble. With these considerations in mind, it should also beunderstood that the steps of performing a cyclic shift as describedabove may be applied to the formation of the SS or the LS within thepre-amble regardless of whether the preamble is for a mixed mode or aGreenfield mode signal.

To illustrate one embodiment of a preamble according to one embodimentof the invention, FIG. 6 illustrates a high data rate pre-amble format(e.g., a modified Greenfield format) for 1-4 spatial streams. As may beseen, the preamble formats shown generally at 300, four preamble formatsare shown at rows 304-316 for 1-4 signal streams that are to betransmitted from a corresponding number of antennas. The short trainingsequence (SS), the first long training sequence (LS1) and the signalfield (SIG-N) are each 8 microseconds long while the each subsequentlong training sequence (LS2, LS3 and LS4) is 4 microseconds long andadds 4 microseconds to the length (period) of the preamble.

To be more specific regarding the preamble lengths, LS1 is 8microseconds (2 symbols+2 added guard interval periods) wherein theguard interval periods are also known as “cyclic prefixes”. LS2-LS4, ifpresent, are each an additional 4 microseconds (1 symbol+1 GI).

Referring now to row 304 of FIG. 6, which illustrates transmission fromonly one spatial stream, a short training sequence SS is followed by along training sequence LS1 and a signal field SIG-N in the describedembodiment of the invention. The short training sequence SS is formedaccording to 802.11(a) specified formats in the described embodiment ofthe invention and carries a small set of frequency tones. Specifically,SS comprises ten tones that are each 0.8 microseconds in length. Thesetones are used by a receiver for one or more purposes includingautomatic gain control for the incoming signals. Long trainingsequences, such as LS1, are for enabling the receiver to perform channelestimation for the associated channel or spatial signal stream. A signalfield, labeled as SIG-N, follows the first long training sequence. Thesignal field is for carrying transmission related information including,for example, number of spatial streams, constellation size and bits persymbol in one embodiment. Each of the SS, LS1 and SIG-N fields(including guard intervals) are eight microseconds long in the describedembodiment of the invention.

The preceding paragraph defines a pre-amble format for a transmission ofone data stream from one antenna. If the transmission is to includeadditional streams from additional antennas, an additional long trainingsequence is required for each additional stream to train a receiverproperly to enable the receiver to process the incoming streamstransmitted from the plurality of antennas. Each additional longtraining sequence adds 4 microseconds of duration to the length of thepreamble. For example, row 316 of FIG. 6 illustrates a field for SS, LS1and SIG-N as well as a field having a 4 microsecond period for eachadditional long training sequence shown as LS2, LS3 and LS4 for atransmission that includes four spatial streams.

In the described embodiment of the invention, four long trainingsequences are generated for transmissions of either 3 or 4 streams. Inan alternate embodiment, only three long training sequences (LS1, LS2,LS3) are generated if only three streams are to be transmitted. Finally,it should be noted that every field and, thus, the total length of thepreamble is equal to a whole multiple of 4 microseconds. Because of realworld variations from design due to process and other variations, anactual period may vary slightly from the specified amounts. Thus, to bemore accurate, each field length each total calculated length isapproximately equal to a whole multiple of 4 microseconds. In thedescribed embodiment, each stream is transmitted at the same frequencyas the other streams and is orthogonal relative to the other streams.Additionally, each stream is cyclic shifted in the time domain toeffectively phase shift each stream relative to each other in thefrequency domain. For example, samples of a second stream may be cyclicshifted relative to a first stream by a first amount. Samples of a thirdstreams may be cyclic shifted by a second amount relative to the firststream. For each subsequent stream, the samples a cyclic shifted by adifferent amount relative to the first stream. Generally, the use ofcyclic shifting avoids tones inadvertent canceling each other bycreating a null at the receiver. Finally, it should be mentioned thatthe entire preamble for a high data rate (Greenfield) transmission istransmitted with beam forming techniques in the described embodiment ofthe invention. While FIG. 6 and subsequent figures may refer to four orless spatial streams, it is to be understood that the principlesdisclosed apply in equal force to preambles for five or more spatialstreams.

FIG. 7 is an example of a mixed mode pre-amble according to oneembodiment of the invention. The preamble for mixed mode transmission inthis embodiment are shown generally at 320 includes a 20 microsecondlegacy prefix that includes a short training sequence SS. A legacydetectable portion 324 of each preamble further includes a SIG-N signalfield that follows the legacy prefix (L-prefix). The SIG-N field isdetectable though not readable by a legacy device. The preamble furtherincludes a short training sequence SS and a long training sequence LS1.Both the legacy prefix and the signal field are shown generally at 324to represent that the signal is legacy device readable. Remaining fieldsshown generally at 328 are readable by high data rate (non-legacy) mixedmode receivers. The second short training sequence shown as “SS” in eachof the rows 332-344 is added to accommodate automatic gain control (AGC)adjustments that may be necessary for beam forming applications. If beamforming is used, beam forming is applied to signal fields that aretransmitted after the SIG-N field in the portion shown generally at 328.In the described embodiment of the invention, SIG-N is modulated usingr=1/2 BCC64 encoding with a +90 degree rotated BPSK modulation using twosymbol durations (8 microseconds). The same modulation is used forGreenfield (high data rate) and mixed mode transmissions for the SIG-Nsignal fields.

As an additional aspect of the present invention, one embodimentincludes a long training sequence having the following characteristicsas shown in Tables 1 and 2. For a single stream, the long trainingsequence is characterized by 0.11aLT as defined in the IEEE 802.11a-1999standards in the described embodiment. Generally, though, any complexsymbol sequence (one constellation point per subcarrier) may beutilized. For a two stream mode of operation, the first stream for firstand second time periods (symbol intervals) may be characterized also by0.11 aLT as shown in Table 1 below.

A second stream, however, is phase shifted relative the first stream andis represented by −0.11aLT*e^(j*θ(t)) or 0.11aLT*e^(j*θ(K)) in the firstand second time periods where K is the subcarrier index. As is shown inTable 3, for a 3 stream mode of operation, the first stream for first,second and third time periods may be characterized also by 0.11aLT*W11,0.11aLT*W12 and 0.11aLT*W13, respectively, as shown in the table. Asecond stream, however, is phase shifted relative the first stream andis represented by 0.11aLT*W21*e^(j*θ(K)), 0.11aLT*W22*e^(j*θ(K)) and0.11aLT*W23*e^(j*θ(K)) wherein, in the first, second and third timeperiods respectively K is the subcarrier index.

A third stream is represented by 0.11aLT*W31*e^(j*φ(K)),0.11aLT*W32*e^(j*φ(K)) and 0.11aLT*W33*e^(j*φ(K)) respectively for thefirst, second and third time periods. “Wmn” refers to DFT Matrixelements and theta (θ) and phi (φ) represent subcarrier phase shifts asis known by one of average skill in the art.

In a system operating in a 4 stream mode of operation, the subcarrierphase shifts are represented by theta (θ), and phi (Φ) and psi (ψ). Asbefore, each column represents a time period. The polarity and phase forthe modulation of the long training sequence for each stream is shown inTable 3. TABLE 1 Long training sequence for two streams Stream firstperiod second period 1 .11aLT .11aLT 2 −.11aLT * e^(j*θ(t)) .11aLT *e^(j*θ(t))

TABLE 2 Long training sequence for three streams Stream first periodsecond period third period 1 .11aLT * W11 .11aLT * W12 .11aLT * W13 2.11aLT * W21 * e^(j*θ(K)) .11aLT * W22 * .11aLT * W23 * e^(j*θ(K))e^(j*θ(K)) 3 .11aLT * W31 * e^(j*Φ(K)) .11aLT * W32 * .11aLT * W33 *e^(j*Φ(K)) e^(j*Φ(K))

TABLE 3 Long training sequence for four streams Stream first periodsecond period third period fourth period 1 −1 * .11aLT .11aLT .11aLT.11aLT 2 .11aLT * −1 * .11aLT * .11aLT * e^(j*θ(K)) .11aLT * e^(j*θ(K))e^(j*θ(K)) e^(j*θ(K)) 3 .11aLT * .11aLT * −1 * .11aLT * .11aLT *e^(j*Φ(K)) e^(j*Φ(K)) e^(j*Φ(K)) e^(j*Φ(K)) 4 .11aLT * .11aLT * .11aLT *−1 * .11aLT * e^(j*ψ(K)) e^(j*ψ(K)) e^(j*ψ(K)) e^(j*ψ(K))

From examining the above, the per carrier phase shifts are implementedwith simple cyclic delays. The above alternate embodiments for high datarate (Greenfield) and mixed mode (legacy) transmissions keeps pre-amblesas short as possible, provides a robust signal field, and supportssimplified receiver processing. Four microsecond boundaries are utilizedfor all fields and orthogonal long training is employed.

One object of a mixed mode transmission is to support high data ratetransmissions is a way that is compatible with legacy devices. As a partof maintaining compatibility, it is advantageous to advise a legacydevice to remain silent while the mixed mode transmitter generatescommunication signals to reduce interference between transmitters. If ahigh data rate transmitter such as a Greenfield transmitter were tomerely transmit, a legacy device may transmit at the same time creatinginterference between associated communication links. Thus,advantageously, the mixed mode transmitter disclosed herein in FIG. 7 isoperable to generate a preamble that enables a different transmitter (alegacy device) to determine to remain silent for the duration of thetransmission of the mixed mode frame. Effectively, the legacy field andthe short training sequence are operable to advise other legacy andnon-legacy transmitters to remain silent. Such protection of thecommunication channel is known as “phi-layer” protection.

FIG. 8 is a flow chart of a method according to one embodiment of theinvention. Generally, the invention includes generating a preamble withan arrangement that depends upon whether a Greenfield (high data rate)or Mixed Mode transmission is to occur and that depends upon a number ofspatial streams that are to be generated. Thus, the method includesgenerating a preamble having an arrangement that depends on whether aGreenfield or Mixed Mode transmission format is to be used and thatfurther depends on how many outgoing data streams are to be transmittedand finally over how many antennas are to be used to carry the outgoingtransmissions (step 350). Thus, for a transmission in which only onestream is to be transmitted on only one antenna, the method includesgenerating a preamble comprising a short training sequence, a longtraining sequence, and a signal field (step 354). If a mixed mode signalis to be transmitted for one or more spatial streams, the methodaccording to one embodiment, includes generating a legacy prefix toadvise legacy devices to not transmit for a specified period. For atransmission in which two streams are to be transmitted on two antennas,the method includes generating a preamble comprising a short trainingsequence, a first long training sequence, a signal field, and a secondlong training sequence (step 358). For a transmission in which threestreams are to be transmitted on three antennas, the method includesgenerating a preamble comprising a short training sequence, a first longtraining sequence, a signal field, a second long training sequence, anda third long training sequence (step 362).

Alternatively, for three streams that are to be transmitted on threeantennas, the method includes generating a preamble comprising a shorttraining sequence, a first long training sequence, a signal field, asecond long training sequence, a third long training sequence, and afourth training sequence (step 366). For four streams that are to betransmitted on four antennas, the method includes generating a preamblecomprising a short training sequence, a first long training sequence, asignal field, a second long training sequence, a third long trainingsequence, and a fourth training sequence (step 370). As may be seen,each of the steps 354 through 370 are alternative to each and generallydepend on the number of streams that are to be produced over acorresponding number of antennas. Additionally, steps 362 and 366 arealternative to each other for three streams and are a matter of designchoice.

The method of the embodiment of FIG. 8 further includes generating phaseshifted and orthogonal long training sequences and generating a signalfield with rotated BPSK signal modulation in one described embodiment ofthe invention. In alternative embodiments, only some of these aspectsare of the invention are included. Thus, one embodiment includes forminga preamble with orthogonal long training sequences (step 374). Theembodiment of the invention further includes forming a preamble having aphase shifted (frequency domain) signal whose phase shift is created bycyclic shifts in the time domain, as well as having orthogonal short andlong training sequences and a signal field (step 378). Finally, theinvention includes generating a signal field (e.g., SIG-N) with rotatedBPSK signal modulation (step 382). In this embodiment, the BPSKmodulation is rotated by a minus 90 degrees (−π/2 radians) to place themodulation onto the quadrature axis. Alternatively, the BPSK modulationmay be rotated by a different amount (e.g., +π/2 radians).

As one of ordinary skill in the art will appreciate, the term“substantially” or “approximately”, as may be used herein, provides anindustry-accepted tolerance to its corresponding term and/or relativitybetween items. Such an industry-accepted tolerance ranges from less thanone percent to twenty percent and corresponds to, but is not limited to,component values, integrated circuit process variations, temperaturevariations, rise and fall times, and/or thermal noise. Such relativitybetween items ranges from a difference of a few percent to magnitudedifferences. As one of ordinary skill in the art will furtherappreciate, the term “operably coupled”, as may be used herein, includesdirect coupling and indirect coupling via another component, element,circuit, or module where, for indirect coupling, the interveningcomponent, element, circuit, or module does not modify the informationof a signal but may adjust its current level, voltage level, and/orpower level. As one of ordinary skill in the art will also appreciate,inferred coupling (i.e., where one element is coupled to another elementby inference) includes direct and indirect coupling between two elementsin the same manner as “operably coupled”.

The preceding discussion has presented a programmable transmitter forgenerating frames of different formats according to different operatingmodes. As one of ordinary skill in the art will appreciate, otherembodiments may be derived from the teachings of the present inventionwithout deviating from the scope of the claims.

1. A transmitter, comprising: a baseband processor operable to generateone or more outgoing digital signal streams for transmission over one ormore antennas substantially at the same time according to a selectedtransmission mode of operation; wherein the baseband processor defineslogic for generating a preamble for each outgoing digital signals streamaccording to whether a high data rate or mixed mode transmission formatis to be used for the outgoing digital signal streams; and wherein, fora high data rate transmission format, the preamble format comprises: a24 microseconds period if only one stream is to be transmitted; and a 24microsecond periods plus 4 microseconds for each additional stream thatis to be transmitted.
 2. The transmitter of claim 1 wherein the formatfor the preamble for one stream includes fields for a short trainingsequence, a first long training sequence, and a signal field.
 3. Thetransmitter of claim 2 wherein the preamble format fields are arrangedsequentially, relative to each other, in the order of short trainingsequence, the first long training sequence, and signal field.
 4. Thetransmitter of claim 2 wherein the preamble format comprises a 28microsecond period and a second long training sequence following thesignal field.
 5. The transmitter of claim 4 wherein the preamble formatcomprises a 32 microsecond period and a third long training sequencefollowing the second long training sequence.
 6. The transmitter of claim5 wherein the preamble format comprises a 36 microsecond period and afourth long training sequence following the third long trainingsequence.
 7. The transmitter of claim 1 wherein each preamble formatcomprises a period that is approximately a whole multiple of 4.0microseconds.
 8. The transmitter of claim 1 wherein, for a mixed modetransmission format, the preamble format comprises: a 36 microsecondperiod if only one stream is to be transmitted; and a 36 microsecondperiod plus 4 microseconds for each additional stream that is to betransmitted.
 9. The transmitter of claim 8 wherein the format for thepreamble for one stream includes fields for carrying legacy information,a signal field, a short training sequence, and a first long trainingsequence.
 10. The transmitter of claim 9 wherein the preamble formatfields are arranged sequentially, relative to each other, in the orderof legacy information, the signal field, the short training sequence,and the first long training sequence.
 11. The transmitter of claim 10wherein the preamble format comprises a 40 microsecond period and asecond long training sequence following the signal field.
 12. Thetransmitter of claim 11 wherein the preamble format comprises a 44microsecond period and a third long training sequence following thesecond long training sequence.
 13. The transmitter of claim 12 whereinthe preamble format comprises a 48 microsecond period and a fourth longtraining sequence following the third long training sequence.
 14. Thetransmitter of claim 10 wherein the first long training sequence is oneof 4 or 8 microseconds long.
 15. The transmitter of claim 8 wherein theshort training sequence and each long training sequence is phase shiftedin the frequency domain by performing a cyclic shift of a series ofsamples in the time domain.
 16. The transmitter of claim 8 wherein thesignal field is generated with a rotated BPSK modulation.
 17. Thetransmitter of claim 8 wherein, for a plurality of streams, each shorttraining sequence is an orthogonal sequence relative to each of theother short training sequences.
 18. The transmitter of claim 8 wherein,for a plurality of streams, each long training sequence is an orthogonalsequence relative to each of the other long training sequences.
 19. Thetransmitter of claim 1 wherein the short training sequence and each longtraining sequence is phase shifted in the frequency domain by performinga cyclic shift of a series of samples in the time domain.
 20. Thetransmitter of claim 1 wherein the signal field is generated with arotated BPSK modulation.
 21. The transmitter of claim 8 wherein, for aplurality of streams, each short training sequence is an orthogonalsequence relative to each of the other short training sequences.
 22. Thetransmitter of claim 8 wherein, for a plurality of streams, each longtraining sequence is an orthogonal sequence relative to each of theother long training sequences.
 23. A transmitter operable to generate apreamble having a preamble format comprising: a first stream utilizing alegacy long training sequence; a short training sequence; a first longtraining sequence a second long training sequence utilizing a phaseshifted legacy long training sequence phase shifted by a first valuerelative to the first long training sequence.
 24. The transmitter ofclaim 23 wherein the preamble format further includes a third longtraining sequence utilizing a phase shifted legacy long trainingsequence phase shifted by a second value relative to the first longtraining sequence.
 25. The transmitter of claim 24 wherein the preambleformat further includes a fourth long training sequence utilizing aphase shifted legacy long training sequence phase shifted by a thirdvalue relative to the first long training sequence.
 26. The transmitterof claim 25 wherein the phase shifts are created by introducing cyclicdelays of first, second and third values of samples in the time domain.27. The transmitter of claim 26 wherein each long training sequence isorthogonal relative to each of the other long training sequences. 28.The transmitter of claim 23 wherein the pre-amble further includes asignal field that defines transmission parameters for outgoingtransmissions.
 29. The transmitter of claim 28 wherein the signal fieldis generated as a rotated BPSK signal.
 30. The transmitter of claim 28wherein every field of the preamble has a period that is approximatelyequal to whole multiple of 4.0 microseconds.
 31. A method fortransmitting a preamble for one of a high data rate or mixed modeoutgoing transmission, comprising: for an outgoing transmissioncomprising only one stream transmitted on only one antenna, generating apreamble comprising a short training sequence, a long training sequence,and a signal field, in that order; for an outgoing transmissioncomprising two streams transmitted on two antennas, generating apreamble comprising a short training sequence, a first long trainingsequence, a signal field, and a second long training sequence, in thatorder; and for an outgoing transmission comprising three streamstransmitted on three antennas, generating a preamble comprising a shorttraining sequence, a first long training sequence, a signal field, asecond long training sequence, and a third long training sequence inthat order.
 32. The method of claim 31 for an outgoing transmissionwherein, for an outgoing transmission comprising three streamstransmitted on three antennas, generating a preamble comprising a shorttraining sequence, a first long training sequence, a signal field, asecond long training sequence, a third long training sequence, and afourth training sequence, in that order.
 33. The method of claim 31 foran outgoing transmission wherein, for an outgoing transmissioncomprising four streams transmitted on four antennas, generating apreamble comprising a short training sequence, a first long trainingsequence, a signal field, a second long training sequence, a third longtraining sequence, and a fourth training sequence, in that order. 34.The method of claim 32 wherein each field within the outgoingtransmission comprises a period that is approximately equal to a wholemultiple of 4.0 microseconds.
 35. The method of claim 31 wherein thepreamble format includes long training sequences wherein each longtraining sequence is phase shifted and orthogonal relative to the otherlong training sequences.
 36. The method of claim 35 further comprisinggenerating a signal field, wherein the signal field is generated as arotated BPSK signal.